Molybdenum
Symbol |
Name |
Atomic Number |
Atomic Weight |
Group Number |
Mo |
Molybdenum |
42 |
95.94 |
6 |
Standard Sate: solid at 298K
Color: grey metallic
Molybdenum is a silvery-white, hard, transition metal. Scheele discovered it in 1778. It was often confused with graphite and lead ore. Molybdenum is used in alloys, electrodes and catalysts.
(Mo), chemical element, silver-gray refractory metal of Group VIb of
the periodic table, used to impart superior strength to steel and other
alloys at high temperature.
A brief treatment of molybdenum follows. For the mining, recovery,
and applications of molybdenum, see Industries, Extraction and
Processing: Molybdenum.
The Swedish chemist Carl Wilhelm Scheele had demonstrated (c.
1778) that the mineral molybdaina (now molybdenite), for a long time
thought to be a lead ore or graphite, certainly contains sulfur and
possibly a previously unknown metal. At Scheele's suggestion, Peter
Jacob Hjelm, another Swedish chemist, successfully isolated the metal
(1782) and named it molybdenum, from the Greek molybdos, "lead."
Molybdenum is not found free in nature. A relatively rare element, it
is about as abundant as tungsten, which it resembles. Most
commercial production is from ores containing the mineral
molybdenite. The concentrated mineral is usually roasted in an excess
of air to yield molybdenum trioxide (MoO3), also called technical
molybdic oxide. Subsequent treatment depends on the ultimate use of
molybdenum. Molybdenum may be added to steel in the furnace in
the form of either technical oxide or ferromolybdenum.
Ferromolybdenum (containing at least 60 percent molybdenum) is
produced by igniting a mixture of technical oxide and iron oxide.
Molybdenum metal is produced in the form of a powder by
hydrogen reduction of chemically pure molybdic oxide or ammonium
molybdate, (NH4)2MoO4. The powder is converted to massive metal
by the powder-metallurgy process or by the arc-casting process.
Molybdenum-base alloys and the metal itself have useful strength at
temperatures above which most other metals and alloys are molten.
The major use of molybdenum, however, is as an alloying agent in
the production of ferrous and nonferrous alloys, to which it uniquely
contributes hot strength and corrosion resistance. It is one of the most
effective elements for increasing hardenability of iron and steel, and it
also contributes to the toughness of quenched and tempered steels.
The high corrosion resistance needed in the stainless steels used for
processing pharmaceuticals and in the chromium steels for automotive
trim is uniquely enhanced by small additions of molybdenum.
Metallic molybdenum has been used for such electric and electronic
parts as filament supports, anodes, and grids. Rod or wire is used for
heating elements in electric furnaces operating up to 1,700 C (3,092
F). Coatings of molybdenum adhere firmly to steel, iron, aluminum,
and other metals and show excellent resistance to wear.
Natural molybdenum is a mixture of seven stable isotopes:
molybdenum-92 (15.84 percent), molybdenum-94 (9.04 percent),
molybdenum-95 (15.72 percent), molybdenum-96 (16.53 percent),
molybdenum-97 (9.46 percent), molybdenum-98 (23.78 percent),
and molybdenum-100 (9.13 percent). Molybdenum exhibits
valences of 2 to 6 and is considered zerovalent in the carbonyl
Mo(CO)6. Molybdenum(VI) appears in the trioxide, the most
important compound, from which most of its other compounds are
prepared, and in the molybdates (containing the anion MoO42-), used
to produce pigments and dyes. Molybdenum disulfide (MoS2), which
resembles graphite, is used as a solid lubricant or as an additive to
greases and oils.
Molybdenum is an essential trace element in plants; in legumes as a
catalyst it assists bacteria in fixing nitrogen. Molybdenum trioxide
and sodium molybdate (Na2MoO4) have been used as micronutrients.
atomic number 42 atomic weight 95.94 melting point 2,610 C (4,730
F) boiling point 5,560 C (10,040 F) specific gravity 10.2 (20 C)
valence 0, 2, 3, 4, 5, 6 electronic config. 2-8-18-13-1 or (Kr)4d55s1
"molybdenum" Encyclop�dia Britannica Online.
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